In various information/signal processing equipment including a network apparatus such as a router, a server, and a host computer, an information/signal processing is under a process of large-scaling and improved in a speed. In such equipment, signal has been conventionally transmitted by electric wirings between CPUs and memories on circuit substrates (boards), between wiring substrates, and between apparatuses (racks). However, from the superiority in view of transmission speed, data transmission capacity, power consumption, radiation from a transmission path, and interference of electromagnetic wave to the transmission path, so-called optical interconnection is actually beginning to be introduced, which transmits the signal by light using an optical fiber or the like as the transmission path, instead of the above mentioned electric wiring. In the optical interconnection, an optical connector has been used for optical coupling of the optical fibers. The typical optical connector has a lens in which the light emitted from an end of one optical fiber is condensed to an end of other optical fiber.
By the way, an amount of the optical communication information rapidly increases in recent years, in addition, long-distance and high-speed transmission of the information are desired. However, in the case of multimode fiber having been conventionally used, the optical fiber having core diameters of 50 μm and 62.5 μm are adopted. Since the multimode fiber transmits the optical signal in a plurality of modes, there is a shift between the attainment times of the signals, resulting in generation of modal dispersion. Thus, due to data loss caused by modal dispersion, the multimode fiber is considered as unsuitable for the long-distance and high-speed transmission.
On the other hand, single mode fiber is an optical fiber having an extremely fine diameter of which a mode field diameter is about 9 μm, and it has an advantage capable of suppressing attenuation as much as possible by transmitting an optical signal in one mode. Accordingly, the single mode fiber has been often used because the attainment time of signal is single, which is different with the transmitting process using many modes such as multimode fiber, thus generating no mode loss, and is suitable for the long-distance and high-speed transmission.
However, the single mode fiber involves one subject due to the small mode field diameter of about 9 μm. That is, when the optical fibers are optically coupled using the optical connector, the allowance of core offset is narrowed. Particularly, the core offset can be caused by a change of environmental temperature. Hereinafter, detail thereof will be explained.
In the typical optical connector, multicores optical fiber bodies composed of a plurality of cores bundled are often coupled for the purpose of increasing the information amount. The optical connector used for such application typically has a holding member to hold the multicores optical fiber body which is called as a ferrule, and an optical element arranged between a pair of ferrules, which has a lens formed for transmitting light effectively between a plurality of core ends held in the ferrule.
However, even if the optical fibers are coupled with high accuracy using the optical connector at normal temperature, a thermal expansion difference may occur in each part by change of the environmental temperature, which may result in an increased loss by the core offset or the like. Here, the optical fiber is made of glass, and the ferrule is often molded from the resin mixed with the glass fiber. The resin mixed with the glass fiber has a characteristic that a coefficient of linear expansion thereof is near to that of the glass, so that the difference of thermal expansions hardly arises. On the other hand, a predetermined optical characteristic is required for the optical element, and hence, depending on the selected material of the optical element, a thermal expansion difference may occur between the optical element and the ferrule. As a countermeasure, in the case of the lens being produced by glass, for example, the coefficient of linear expansion of the lens glass is near to that of the optical fiber and the ferrule, which is costlier.
On the other hand, as disclosed in Patent Document 1, the optical element is produced by resin containing glass fiber, thereby to suppress the linear expansion difference between the optical element and the ferrule, which can suppress the efficiency loss on change of the environmental temperature. However, in the material produced by containing the glass fiber to the resin, there exists a technical difficulty to match the refractive index and the temperature characteristic to a level satisfying the optical characteristic. Also, the resin is generally colored by mixing the glass fiber, which may reduce the optical transmittance. Therefore, it is difficult to use the resin containing the glass fiber for the optical element used for optical connector.